This invention relates generally to systems for sensing a condition at a plurality of locations and more particularly to systems for conditioning multiple sense elements with a common electronic circuit.
Many applications call for the sensing of a condition, such as pressure, acceleration, torque and force, at a plurality of locations. By way of example in the automotive environment, electro-hydraulic brake or EHB systems generally have six locations that require sensing of the fluid pressure. Pressure sensing is required at each wheel for closed loop brake force control, at a location to sense driver input and at the pressure accumulator to sense system reserve pressure. Hydraulic sensing points are all routed through the hydraulic control unit or HCU having a system controller, i.e., microprocessor, so that there exists one member at which all different hydraulic circuit pressures are sensed. The provision of six discrete pressure sensors with full conditioning electronics results in suitable operation; however, it also results in a total pressure sensor cost which is higher than desirable compared to the remaining system component costs.
It is therefore, an object of the present invention to provide a reliable yet lower cost condition responsive sense system than the prior art system referenced above. Yet another object is the provision of a relatively low cost, accurate and reliable sense system responsive to pressure, acceleration, torque, force and the like and to an improved low cost method for conditioning condition responsive sense elements.
Briefly stated, in a condition responsive sense system made in accordance with the invention, a plurality of sense elements are connected to an ASIC with the output of any selected sense element being connected to a common signal conditioning circuit path by analog multiplexing. The sense element signal is conditioned by the signal conditioning circuit of the ASIC to provide partial conditioning comprising basic calibration data. Complete characterization data for all the sense elements is stored in non-volatile memory of the ASIC and is transferred to a host controller, e.g., microprocessor, upon command to enable the host controller to perform appropriate mathematical operations to provide the additional amount of compensation required to complete the signal conditioning. According to a feature of the invention, a diagnostic circuit path is included in the ASIC for diagnosing sense element and sense element connection faults. According to another feature, first and second fixed test sense elements are formed in the ASIC for diagnosing ASIC faults.
According to a preferred embodiment of the invention, the sense elements may be formed of individual strain bridges known in the art, such as silicon resistor bridges bonded to a diaphragm through glass material fired at high temperatures and adapted for placement in a fluid pressure port. When pressurized, fluid is present in the pressure port and the diaphragms and bridges will undergo elastic strain. Because the resistors in the bridge are made of silicon, they exhibit a piezoresistive effect exhibiting a change in resistance when subjected to strain. By applying a voltage to the bridge, a small voltage change results at the output of the bridges. In the described embodiment, a circuit for six sense elements is shown; however, it will be realized that the circuit can be modified to accommodate more or fewer sense elements as desired.
According to the preferred embodiment described, a bridge bias is provided through an electronically programmable resistor in series with the strain bridge. The resistor can be set either to a specific value in order to derive a temperature signal from the bridge or it can be set at zero to bias the bridge with the full supply voltage. Selection of the bias resistor value is accomplished via selection of a register value. Generation of the temperature signal utilizes the bridge temperature coefficient of resistance (TCR) to form a voltage divider with the low/zero TRC programmable resistor. Derivation of the temperature signal by this means is conventional.
In accordance with the invention, the bridge conditioning circuit functions with one bridge at a time, therefore, a means of switching bridge bias and each bridge output to the conditioning circuit input is provided. This is accomplished via analog multiplexers. The input to the conditioning circuit is controlled by three register values. All bridges are connected to a terminal of the ASIC at all times. In addition to the six pressure sensor inputs, two reference bridges are also included in the circuit and are selected via the analog multiplexer registers for the purpose of circuit diagnostic testing. The function of the reference bridges will be explained more below.
Electronically programmable Offset and Gain correction of the signal proportional to pressure is provided through control of respective registers. This minimal amount of calibration is required to maximize the output range of the pressure signal in order to use the largest possible input range of the Analog to Digital Converter (ADC). By using this approach, the bit resolution of the ADC block can be reduced to the minimum acceptable level thereby reducing the size and cost of this circuit element. Optional input Low Pass Filters are provided to attenuate high frequency noise sources (e.g., EMI), and optional low pass filters at the output of the electronically programmable gain stages are provided to tailor the system to meet customer frequency response characteristics.
Diagnostic functions are implemented through two portions of the circuitry. The first is a sense element diagnostic circuit which:
Provides an offset corrected and amplified signal proportional to the addition of the selected bridge output signals. Ideally, the output of the sense element diagnostic circuit is independent of pressure and temperature and, therefore, changes in this parameter can be used to indicate sense element failures (e.g., bridge parameter drift or hard failure). System comparison of the compensated output, with the value stored at the time of manufacture, is performed to determine if the sense element performance has degraded. Due to manufacturing tolerances, there will be a pressure and temperature dependence, which will reduce the accuracy of the sense element diagnostic signal and reduce system level error detection capability. System use of the pressure and temperature signal from the selected bridge provide a means of correcting pressure and temperature related sense element diagnostic signal errors allowing for more accurate analysis and finer error detection capability.
The second portion of the circuitry for providing diagnostic function, first and second reference bridges:
The function of the reference bridges is to input a fixed bridge signal proportional to 0 and full-scale bridge outputs, respectively. Pressure, temperature and sense element diagnostic signals are calibrated at the time of manufacture and stored in ASIC Non-Volatile memory. When the test modes are activated, the system can analyze the pressure, temperature and sense element diagnostic output signals, compare the outputs with the values stored at the time of manufacture, and determine if circuit performance has degraded or malfunctioned. Use of the 0 and full-scale reference bridges fully exercises the full operating range of all circuit paths providing extensive diagnostic capability.
A digital port function is provided to transmit data to and receive data from the host or system controller. The system controller inputs a command to the conditioning electronics, which in turn, performs the function requested by the controller. For example, the controller may request pressure information from sense element 2. The Digital Port receives the command and a Logic Control block verifies (e.g., parity check) and decodes the command. The Logic Control block then sets the correct multiplexer switch positions, loads the corresponding register values, waits for the analog signal path to stabilize, triggers an analog to digital conversion, loads the output of the analog to digital converter into the digital port, and instructs the Digital Port to transmit the requested information. Combined instructions are implemented to provide more than one piece of information transmitted at a single time (e.g., pressure, temperature and sense element diagnostics acquired and transferred in a single transmission), or the system can be instructed to continually transmit a sequence of data (e.g., P1, SDC1, P2, SDC2, P3, SDC3, P4, SDC4, P5, SDC5, P6, SDC6, T1, repeat . . . ). The format of digital data transmission can be customized to meet the customer requirements.
To minimize the complexity and cost of the system, only the minimum amount of electronic calibration of the pressure, temperature and diagnostic signals are provided. The rest of the signal correction will be accomplished with the existing system controller (e.g., microprocessor) through the use of additional coefficients stored in the ASIC. At calibration, the circuit is electronically calibrated via control of the binary register values and all digital settings are stored in Non-Volatile memory in the ASIC (e.g., EEPROM). The sense elements and circuit outputs (pressure, temperature and sense element diagnostics) are then characterized as an assembly over pressure and temperature to determine the operation over the full operating parameter range. From this test data, coefficients suitable to mathematically correct the remaining temperature, pressure and diagnostic signal errors are determined and stored in the ASIC Non-Volatile memory. The conditioning electronics has the ability to communicate the compensation coefficients to the system controller so that it can utilize its existing capabilities; to perform high accuracy, mathematical correction of the pressure, temperature and diagnostic signals.